1. Field of the Invention
The present invention relates to the preparation of plastic articles by injection molding. More particularly, this invention relates to methods and apparatus useful in the gas-assisted injection molding art. New and improved injection molds or tools and methods for making such molds or tools have been discovered whereby molds suitable for injection molding, or gas-assisted injection molding, or other types of low pressure molding can be prepared much quicker and more economically than by conventional mold-building techniques and procedures.
2. Description of the Prior Art
Initially, injection molding involved the injection of a molten thermoplastic material into a relatively simple mold cavity, allowing the thermoplastic material to cool to form a molded article, and then opening the mold cavity to release the article. The articles formed were relatively simple in form and shape; the molds, therefore, were also relatively simple in both design and construction.
As the art advanced, however, it became desirable to manufacture larger and more complicated parts, including those having thick rib sections and complicated cross-sections. In addition, gas-assisted injection molding was found to be particularly suited for the manufacture of such complicated articles. Indeed, the development of gas-assisted injection molding has allowed for the preparation of even larger and more complex articles.
To fabricate such increasingly complex parts, injection molds have, of necessity, become more and more complex in both design and construction. Today, the design and construction of such complex injection molds represent a considerable expenditure of both time and capital.
Injection molds are generally prepared by one of two methods. In the first general technique, a steel block of the appropriate size is carefully machined to the desired shape, usually using numerical control machine (NCM) techniques. Such NCM techniques involve computer-driven machine tools. Hand finishing is required on ridges and other such complex features. A complicated mold surface can often take two to six months or more of very demanding and costly machining effort. And mistakes at any step of this procedure can, and often do, ruin the mold, thereby requiring the mold to be scraped and the process started anew. In addition, interconnecting channels for cooling must be drilled through the steel block, as well as channels for ejector pins, lifters, and the like. As is apparent to those skilled in the art, construction of such a mold requires considerable effort and, therefore, represents a large investment. Completed steel molds, however, are generally of high quality since the steel mold surfaces can take and hold a fine finish through repeated molding cycles.
In the second general procedure for the manufacture of injection molds, a model having the positive image of the desired article is prepared from wood or epoxy resin. Molten aluminum or zinc are then used to cast the model to produce the desired cavity. Although this procedure is generally faster and less expensive than the steel mold procedure just described, the procedure and the resulting cast mold can have significant limitations. Generally, this process cannot be used to create molds with complex features. Further, the aluminum/zinc molds do not have very good wear properties. Thus, the number of molded articles that can be prepared from such a cast mold is significantly reduced. Additionally, it is more difficult to polish the aluminum/zinc surface in order to obtain a smooth or "class-A" surface on the molded product. Furthermore, the use of molten aluminum and zinc baths involve considerable health and safety considerations.
It is desired, therefore, to provide injection molds and methods for constructing such molds which avoid many of the problems associated with the prior art techniques of mold construction. A mold which could be prepared relatively quickly and at a lower cost would be particularly desirable. It is also desirable that such a mold have a surface which can accept a polished or grained finish and that can withstand repeated injection cycles. The present invention provides such molds as well as methods for their production.